The present invention relates generally to a communications system and, more specifically, to an apparatus and method for implementing a communications interface protocol using existing Cellular Digital Packet Data (xe2x80x9cCDPDxe2x80x9d) transmission equipment.
Cellular Digital Packet Data (xe2x80x9cCDPDxe2x80x9d) is a wireless communications protocol that folds streams of data into envelopes or packets that are transmitted at very high speeds during pauses in cellular phone conversations. This permits the use of existing cellular systems as a means of data transmission. CDPD allows data files to be assembled into packets and transmitted via idle channels of existing bandwidth. Data can be transmitted at 19.2 Kbps over an enhanced cellular network. Adding CDPD to an existing analog cellular system allows cellular systems to transmit data eight times faster without the necessity of creating a completely new digital system.
In practice, packet data is transmitted in a wireless mode using the available bandwidth the Advanced Mobile Phone Service (xe2x80x9cAMPSxe2x80x9d) which operates as the communications infrastructure for analog cellular radio. Digital cellular is referred to as D-AMPS. CDPD specifications are published through the CDPD Forum and follow OSI (Open Systems Interconnection Protocol) guidelines. CDPD technology provides connectivity up to the network layer and is an overlay system that operates on AMPS frequencies.
The RS232 serial interface provides a serial data connection between two devices over dedicated wires. The interface defines up to 26 lines between two devices. One line carries the data and the others carry signaling information. Signaling is achieved by the lines through binary states, either xe2x80x9cONxe2x80x9d or xe2x80x9cOFF.xe2x80x9d Some lines are defined for data and some for signaling. Thus, data transmission can be controlled simultaneous by both the sending device and the receiving device.
For example, the sending device can query the receiving device as to whether it is ready to receive data by setting the signal high on a line called Ready To Send (xe2x80x9cRTSxe2x80x9d). The receiving device can, in turn, reply that it is ready to receive data by setting the signal high on the Clear To Send (xe2x80x9cCTSxe2x80x9d) line. After these conditions are satisfied, by devices can begin data transmission over the data lines a communication speeds.
As noted above, wired RS232 applications conduct data communications between a sender and a receiver that are hard wired together. In a wired RS232 connection based application, several lines may simultaneously be used to send signaling information; the signals on each line are detected by the sender and receiver instantaneously. On the other hand, in wireless RS232 communications, instantaneous communication does not occur. Rather, information is transmitted with a delay of ranging from 0.5 to 4 or 5 seconds. Furthermore, in wireless communications, modems can connect to a large number of devices using the call establishment methodology programmed into the modems.
The present invention provides a method and apparatus for using wireless modems in an RS232 mode, that provide (1) logic establishing the sender and receiver and (2) a signaling and data transmission methodology that makes the wireless mode transparent to the applications. The method and apparatus of the present invention allows utilization of wired RS232 based applications transparently on wireless communication systems, and alleviates the delay inherent in wireless communications without impacting the reliability of the system.
The invention provides a wireless point-to-point communications system for reliable and efficient digital data transfers as compared to prior art network interface protocols. In this regard, and in accordance with one embodiment, the invention utilizes a commonly available communications protocol, such as RS232, or other accepted serial standard to encapsulate digital data derived from an instrument, data collector, or other signal acquisition means in a wireless signal carrier. The serial protocol has built-in error correction and flow control. The end-to-end interface is compatible with ordinary devices supporting the protocol. In addition, the cellular communications infrastructure provides a reliable backbone with call switching and routing of the data to its intended destination. The RS-232 data and flow control signal are encapsulated in a CDPD courier supported in existing cellular infrastructures. Since RS-232 is widely accepted and supported by a plurality of data acquisition processing systems, the interface protocol provided is reliable and efficient.